A series of effective methods and apparatus for multi-level imaging, such as found in the Xerox Corporation 4850 highlight color printer, are shown in U.S. Pat. Nos. 4,078,929, 4,868,600, 4,959,286, 5,049,949, and 5,245,392 (the disclosures of which are hereby incorporated by reference herein). These patents teach a tri-level process which utilizes a modulated laser beam to produce three distinct electrical levels of the latent image on an imaging belt (or other charge retentive surface). Most laser printers use a two level system, or simply an on/off for each imaged pixel. The above mentioned patents, however, use a writing laser beam that is time modulated, the electrostatic latent image being created by the laser beam imaging to three states which are full-on, half-on, and off.
While the multi-level imaging provided by the above mentioned patents is advantageous in a number of ways, there are several disadvantages associated therewith. For example U.S. Pat. No. 4,959,286 points out that a trade-off in using a tri-level imaging in lieu of two-pass imaging is the necessity of imaging three light levels within one frame (i.e. black, white, and colored), thereby cutting the voltage latitude in half or more. This necessitates using a high gamma development system such as a conductive magnetic brush. The toner developing stations utilized in such systems are the result of a considerable expenditure of technological effort and money. The developing systems are dual component toning systems which are complex and sophisticated both to construct and to operate. The voltage difference between the image and donor rollers in each case is about 300 volts, therefore making it necessary to use a variety of different types of techniques in order to achieve adequate image development. For example U.S. Pat. No. 5,245,392 uses a special biased wire to help form a powder cloud of charged toner above a donor roller to improve image development density. The utilization of dual donor (applicator) rollers in these systems is particularly expensive and disadvantageous.
According to the present invention it has been found that by utilizing a non-magnetic toning process and apparatus, such as disclosed in Canadian published patent application 2059036 (based upon U.S. application Ser. No. 07/639,360 filed Jan. 8, 1991, the disclosure of which is hereby incorporated by reference herein), a high gamma development system is provided which is significantly simpler than the prior art development systems for tri-level systems. The development system and method according to the present invention also have far fewer limitations on process speed and variability of process than in the conventional tri-level systems described in the above mentioned U.S. patents because the image potential of the toner is higher, and there is less dependence on toner charging and on the chemical make-up of the toner, providing an enhanced performance window of the tri-level (or other multi-level) imaging process.
According to one aspect of the present invention a method of forming images using first and second fluidized beds of non-magnetic toner, having first and second, respective, applicator rollers is provided. The method comprises the steps of substantially continuously: (a) Uniformly charging a charge retentive surface to a predetermined voltage level. (b) Forming on the charge retentive so surface at different locations thereof at least first and second different, spaced, latent electrostatic images. (c) Moving the charge retentive surface past the first fluidized bed of non-magnetic toner, so that the charge retentive surface comes into operative association with the first applicator roller of the first fluidized bed. (d) Electrically biasing the first fluidized bed and the first applicator roller at a first bias level effective so that the first image is developed by non-magnetic toner transferred from the first applicator roller to the first image while development of the second image is precluded. (e) Moving the charge retentive surface past the second fluidized bed of non-magnetic toner, so that the charge retentive surface comes into operative association with the second applicator roller of the second fluidized bed. And, (f) electrically biasing the second fluidized bed and the second applicator roller at a second bias level, different than the first bias level of step (d), effective so that the second image is developed by non-magnetic toner transferred from the second applicator roller to the second image while development of the first image is precluded.
Typically each fluidized bed has a single applicator roller rather than the dual rollers provided in the prior art, greatly simplifying the system and method. In that case step (c) is practiced by bringing the charge retentive surface into operative association with the first, single, applicator roller, and step (e) is practiced by bringing the charge retentive surface into operative association with the second, single, applicator roller.
Typically step (d) is practiced by applying an electrical bias to the first fluidized bed of toner of a first polarity, and step (f) is practiced by applying an electrical bias to the second fluidized bed of toner of a second polarity. For example step (d) may be practiced to apply a negative electrical bias to the first fluidized bed, and step (f) to apply a positive electrical bias to the second fluidized bed. Steps (c) and (d) may be practiced to apply and develop black toner while steps (e) and (f) apply and develop colored toner. Step (d) may be practiced to apply a first positive voltage to the first applicator roller and step (f) to apply a second positive voltage to the second applicator roller, the second voltage at least 50 volts lower than the first voltage. For example step (d) may be practiced to apply a negative voltage of about 6700 volts to the first fluidized bed and a positive voltage of about 550 volts to the first applicator roller, while step (fi is practiced to apply a positive voltage of about 7600 volts to the second fluidized bed and a positive voltage of about 450 volts to the second applicator roller.
According to another aspect of the present invention image forming apparatus is provided comprising the following components: A movable charge retentive surface. Means for uniformly charging the charge retentive surface to a predetermined voltage level. Means for forming on the charge retentive surface at different locations thereof at least first and second different, spaced, latent electrostatic images. First and second spaced fluidized beds of non-magnetic toner, having first and second, respective, applicator rollers. Means for moving the charge retentive surface first past the first fluidized bed of non-magnetic toner, so that the charge retentive surface comes into operative association with the first applicator roller, and then moving the charge retentive surface past the second fluidized bed of non-magnetic toner, so that the charge retentive surface comes into operative association with the second applicator roller. Means for electrically biasing the first fluidized bed and the first applicator roller at a first bias level effective so that the first image is developed by non-magnetic toner transferred from the first applicator roller to the first image while development of the second image is precluded. And, means for electrically biasing the second fluidized bed and the second applicator roller at a second bias level, different than the first bias level, effective so that the second image is developed by non-magnetic toner transferred from the second applicator roller to the second image while development of the first image is precluded.
The first applicator roller preferably comprises a single applicator roller associated with the first fluidized bed, and the second applicator roller comprises a single applicator roller associated with the second fluidized bed. The movable charge retentive surface may comprise a web, cylinder, or other conventional structure, preferably comprising a movable photoconductive belt. A transfer roller is typically provided between each fluidized bed and its associated applicator roller. The means for uniformly charging the charge retentive surface to a predetermined voltage level typically comprises a corona discharge device, such as a scorotron, corotron, or dicorotron, while the means for forming the charge retentive surface at different locations comprises a laser based output scanning device. All the other conventional components associated with a printer, such as means for transferring the first and second images from the charge retentive surface to a sheet of paper, cleaning apparatus, and the like, are also preferably provided.
According to still another aspect of the present invention, an image forming apparatus is provided comprising the following components: A movable charge retentive surface. Means for uniformly charging the charge retentive surface to a predetermined voltage level. Means for forming on the charge retentive surface at different locations thereof at least first and second different, spaced, latent electrostatic images. First and second spaced fluidized beds of non-magnetic toner. The first fluidized bed having a first, single, applicator roller. The second fluidized bed having a second, single, applicator roller. Means for moving the charge retentive surface first past the first fluidized bed of non-magnetic toner, so that the charge retentive surface comes into operative association with the first applicator roller, and then moving the charge retentive surface past the second fluidized bed of non-magnetic toner, so that the charge retentive surface comes into operative association with the second applicator roller. Means for electrically biasing the first fluidized bed and the first applicator roller. And, means for electrically biasing the second fluidized bed and the second applicator roller. The details of this apparatus are basically the same as described for the preceding embodiment.
It is the primary object of the present invention to multi-level imaging method and apparatus having a significantly enhanced performance window, and greater simplicity than is conventional. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.